It is a term that has been used rather loosely for a long time, but
broadly speaking it is a display technology that has all the attributes
of paper but can be written to and erased electronically. We can list
some of these basic attributes as follows:
- High resolution (150dpi or better).
- High contrast, equal to that of print on paper (about 10:1 or better).
- Readable in any ambient light conditions
- Readable at any viewing angle
- Excellent ergonomic features, easy to hold, carry, and use.
- Light weight, at most comparable to an equal sized sheet of card.
- Robust, will withstand being dropped, hit, etc.
- Flexible, or at least bendable.
- Bistable, once a display is written it will stay displayed even
when power is switched off.
- Cheap, maybe not as cheap as paper, but easily affordable by everyone.
- Reasonable large area, preferably A4 (298x212mm)
A display that meets all of these attributes can be referred to as an e-paper
display suitable for use in an e-publication reader, since it is, in virtually
all aspects, an electronic replacement for a sheet of paper. Indeed such display
technologies are sometimes referred to as paper replacement technologies.
Flexible, bendable or rigid?
Although a lot of emphasis is placed upon e-paper being either bendable or flexible
these are in many ways some of the least important attributes of e-paper. However,
what is important about these attributes as opposed to a rigid glass based display
like an LCD panel, is that their flexibility makes them much more robust and
A rigid glass substrate LCD display will break if dropped on a hard surface,
trodden on, sat upon, etc. A bendable display will probably survive most of those
accidents. A bendable display panel can also be made much thinner and lighter
than a rigid one since it needs no strong physical support to protect it, and
so if it is bent when shoved into a briefcase it will survive.
Because it needs no rigid backing a flexible display panel is thin and light
weight, and hence it is both highly portable and ergonomically much easier to
Our potential user surveys have indicated that the majority of users will settle
for a device that is slightly bendable, rather like a thick sheet of card or
a thin sheet of plywood. An acceptable format would be light enough to easily
hold in one hand for long periods, rigid enough to act as a writing pad for handwritten
annotations using the touch sensitive surface, and yet flexible enough to survive
most forms of mistreatment.
The first generation of e-paper display that are now appearing in the marketplace
all use rigid glass backplanes that are basically derived from conventional LCDs,
this means that they are as rigid and breakable as an LCD. From the second generation
onwards all e-paper displays will at least be bendable, these displays are entering
the manufacturing phase now.
Highly flexible displays will, in our opinion, be confined to specialist niche
applications where large display areas are required by small portable devices:
for example a small pocket GPRS system with roll out map display. Another area
where highly flexible displays will find an application is in wearable data display
systems for military and other use. In the more distant future we may see a number
of highly flexible e-paper displays bound together to form the electronic equivalent
of a book, with display control and data storage electronics in the spine.
The importance of readability.
Although the information storage and distribution function of paper is increasingly
being replaced by digital technology, paper still holds pre-eminence when it
comes to reading that information. By and large most people still prefer to read
from a sheet of paper than from a computer screen. Indeed the much heralded 'paperless
age' of the personal computer has instead been an age where paper usage has been
higher than ever.
The reason for this is that most people do not like reading from a computer screen,
either an LCD display on a laptop, or a CRT screen on a desktop. There are several
reasons for this, the most important are:
- Low contrast ratio and low resolution lead to eyestrain in long
periods of continuous reading off a computer screen.
- The size, and weight of a computer screen means that the reader
cannot easily position himself/herself at a proper viewing distance,
leading to further eyestrain.
- Computer displays are light generative, or backlit, and often not
viewable in a wide range of ambient light conditions or viewing angles,
leading to further eyestrain.
- Lack of portability, even with a laptop, limits the times and places
in which a document can be read off screen.
- The landscape format of a computer display contrasts to the portrait
format of most printed paper documents, resulting in the need for page
scrolling of documents that are formatted for print.
Although some people, especially younger computer users, are happy
to read from screen for long periods, most users find that the above
reasons limit the time that they can comfortably spend reading off screen.
Indeed, the problem is sufficiently serious to be recognised by health
authorities, and in the UK, the normal fee for eye tests can be waived
for computer users.
This means that reading from a screen is usually confined to quick scan
reading and searching for information, rather than careful in depth reading.
Consequently most will opt for printing out a page that they wish to
Why E-paper offers improved readability.
In all computer displays, including e-paper, the display is made up from
a number of very small picture points, or pixels, the image on the screen
being formed by the pattern in which they are turned off and on. Most
conventional computer displays in use today have a resolution of between
70 and 100ppi (pixels per inch). A standard laser or ink jet printer
will print using a resolution of between 300 and 600ppi.
At an average viewing distance of about 60cms a screen resolution of
100ppi gives a fairly sharp image, however, at a closer distance, such
as the 30cms average viewing distance when reading a printed sheet of
paper, the digitisation becomes noticeable, thus reducing both the quality
of the typography and the readability of small and/or serif fonts.
This means that paper replacement displays which will be viewed at a
closer distance will need to have a resolution of at least 150ppi and
preferably 200ppi for a monochrome display if it is to equal the quality
of standard newsprint, and 300ppi or better if it is to equal the quality
of magazine and book printing. Most e-paper technologies are well able
to deliver resolutions up to 300ppi and many have already been demonstrated
The clarity of printed text also depends upon the contrast ratio between
the respective reflectivity of the paper and the ink. In newsprint the
contrast ratio is typically around 10:1, though in higher quality magazines
and books it can be much higher. A typical LCD display, however, will
only have a contrast ratio of about 5:1. In general the higher the contrast
ratio of a display the easier it is to read text based information, and
the aim of any text display technology should be to aim for a contrast
ratio at least equal to that of printed paper. Most e-paper technologies
achieve a better than 10:1 ratio, which is about the same as that of
a printed newspaper.
With colour computer displays the problem is more complex since each
pixel consists of a triad of different coloured pixels, one red, one
blue, and one green, the combination of these three colours together
with the intensity of each will determine the resultant colour of the
pixel triad. The use of such pixel triads means that the overall resolution
of colour displays is usually much lower than that of monochrome displays
- so, since they require three times as many pixels, a 150ppi display
will require 450 colour points per inch.
However, in print, a four colour combination is used: cyan, magenta,
yellow and black, which gives the high contrast black that is necessary
for text, whilst at the same time offering the colour triad to generate
a full colour palette. Although full colour e-paper is not yet in production,
it will probably follow the four colour system used in print rather than
the three colour system used in computer displays if it is to have the
necessary contrast for displaying good quality text.
The readability of text, in particular the contrast between ink and paper,
is also very dependent upon the ambient light conditions and the viewing
angle. In a conventional CRT display, which is light generative, or a
LCD display which is backlit and transmissive the display is easily 'washed
out' in very bright ambient light.
However, in bright light a sheet of printed paper becomes easier to read
because it is being read by reflected light. In general the human eye
finds it far easier to read using reflected light than any form of light
generative/backlit display. Most e-paper technologies use reflective
displays, and this will probably be a major factor in their popularity
since this type of display will generate considerably less eye strain.
Another readability factor where e-paper technologies will have an advantage
is the viewing angle. Both CRT and LCD screens need to be viewed almost
straight on, look at them from an angle and in the case of a CRT one
sees reflections of the room, or in the case of LCD the contrast simply
disappears. A sheet of paper can, however, be viewed at virtually any
angle, the same applies to most of the e-paper technologies.
Why E-paper offers improved usability.
A thin lightweight display has considerable ergonomic advantages over
the conventional LCD and CRT displays available today. The projected
weight of an A4 e-paper display based document reader, including battery,
will be under 200gms, about the same weight as a magazine like the Economist.
This means that it can be comfortably held in one hand and read in any
position or location that the user wishes.
The light weight of an e-paper display based reader device, coupled with
the fact that it will probably be keyboardless (relying instead on a
touch screen and virtual keys) also means that like a sheet of paper
it can be easily used in either landscape or portrait mode. Indeed e-paper
displays have another advantage in that they can be more easily manufactured
in a wide range of sizes and shapes for specialist display applications.
Another ergonomic advantage of e-paper displays is that because they
are reflective and offer a high contrast they can be read in any ambient
light condition that will allow a paper document to be read.
The low power consumption and bistability of an e-paper display means
that they can be used for long periods without recharging or replacing
batteries. Manufacturers of first generation e-paper display based readers
are quoting figures of 10,000 page displays on two AA batteries,
or about three months of average use.
When an e-paper display is combined with a touch screen overlay the combination
offers the capability of becoming an exact electronic analog of a pad
of paper and a pencil. It will be possible to draw handwritten notes
or diagrams onto the display, thus allowing manual annotation of printed
material, note taking etc.
A more in depth look at some of the enabling technologies that are
being used to build e-paper systems.
Electrophoresis is a process originally developed for medical/biological
purposes, allowing the separation of molecules, according to their size
and electrical charge, by applying electric current to them.
In an electrophoretic frontplane small, sub-micron, ink particles are
given an electric charge and then suspended in a dielectric fluid medium
that is encapsulated into a sub-pixel size cell or microcapsule. When
an electric field is applied across this cell or capsule the ink particles
will move towards the electrode with the opposite charge.
If the electrode is transparent then that surface of the cell or capsule
will assume the colour of the ink when the field is applied.
The contrast can be enhanced by taking ink particles with opposite colours
- black and white - and charging them with opposite polarities. Mixed
together in a capsule, when an electric field is applied, all the black
particles will migrate to one side, and all the white to the other. Switch
the field, and the capsule will change colour.
This makes it possible to switch between all black particles and all
white particles on the transparent front electrode of the cell or microcapsule
and thus assure the high contrast ratio that is a feature of electrophoretic
Another important feature of electrophoretic frontplane displays is that
they are bistable because once the particles have migrated towards an
electrode they will stay there, even when the power is switched off.
This means that power is only required to change a cell from white to
black or vice versa, not to maintain it, hence the term bistable. This
is a very important feature since it allows large displays to be used
in portable battery powered devices, without need for constant battery
The difference between the various different electrophoretic frontplane
technologies lies simply in how the charged ink particles and fluid medium
are encapsulated to give a sufficiently fine pixel resolution.
The big drawbacks with the current generation of electrophoretic displays
are firstly that they have slow refresh times, since the pigment particles
take time to move, and secondly that they are monochrome. Both problems
are the subject of considerable research effort and look set to be overcome
in the not too distant future.
A bichromal frontplane consists of a very thin sheet of flexible plastic
containing a layer of microscopic plastic beads, each one encapsulated
in a little pocket of oil and thus able to freely rotate within the plastic
sheet. The two hemispheres of each bead are coloured differently (hence
the term bichromal) and each hemisphere is given a different charge.
When an electric field is applied across the sheet all the beads will
rotate so that the hemisphere with the negative charge on each bead is
next to the electrode with the positive charge, and vice versa. If the
electrodes used to create the field are transparent then the colour of
the sheet will reflect the polarity of the electrode. Reverse that polarity
and the colour of the sheet under the electrode will change to the other
An important feature of bichromal frontplane displays is that they are
bistable. This means that power is only required to change the display,
not to maintain it. This is a very important feature since it allows
large displays to be used in portable battery powered devices, without
need for constant battery recharging.
The disadvantages of bichromal displays is firstly that they are monochrome,
and secondly that the display resolution is limited by the bead size.
However, both these problems are currently being addressed by ongoing
research and should, according to the researchers, be solved in the not
too distant future.
The LCD screen is over thirty years old and is now a well understood
technology. However, developing the technology for the frontplane of
a flexible display brings considerable challenges. As we indicated in
the introduction, the frontplane in a conventional LCD unit is made of
a rigid piece of glass, in order to ensure that the cell gap between
it and the backplane is precisely maintained. Even slight variations
in this gap will produce image distortions.
Maintaining such a precise gap in a rollable or bendable display is extremely
difficult, and requires the use of spacers to maintain the gap. Creating
such spacers is primarily a manufacturing technology problem, and is
one that several companies around the world, including IBM, and Philips
are understood to be now addressing. Two companies, HP and Fujitsu, have
demonstrated actual devices.
Many industry analysts, the authors of this report included, believe
that flexible LCD technology will take a major part of the flexible/bendable
display market sometime during the period 2010 to 2020. The technology
behind LCDs is well understood and the manufacturing techniques needed
to solve the 'gap maintenance' problem are already being developed. In
the medium term flexible LCD technology will give high speed, full colour,
high resolution displays, and thanks to recent developments like cholesteric
LCDs, these displays will have fairly low power consumption and feature
both bistability and reflectivity.
With cholesteric LCDs the liquid crystal molecules form spirals, which
means that incoming light is reflected or transmitted depending on the
spiral's axial direction, which changes in line with the height and length
of voltage pulses. Light is reflected, if the spiral's axes align in "the
which is a direction of the paper's thickness, while light is transmitted,
if the axes align in "the focal conic state," which is a direction
perpendicular to the planar state.
The use of a cholesteric liquid crystal means that the display has a
far better readability than a display using conventional nematic liquid
crystals and can be made thinner, since it reflects 50% of certain wavelengths,
removing the need for colour filters and polarizing layers. This in turn
means that the colour is more vivid, and the contrast much better, than
conventional reflective-type LCDs.
Organic semiconductor backplane
Developments during the 1980s in conductive polymer technology, primarily
at the Cavendish Laboratory in Cambridge, have led to the discovery of
a range of different polymers, in other words plastics, with conductive
and semi-conducting properties. Like their inorganic equivalents these
special materials can be used to build working active electronic circuits,
in particular transistors, memory cells and logic gates, all the basic
building blocks of digital circuitry.
This technology was spun out of Cambridge University in 2000 as Plastic
Logic, a company given the task of turning this basic research into commercial
products. Plastic Logic may be one of the leading companies and a technology
pioneer in plastic electronics but they are not alone. Other companies
who are working in this area include Philips, which recently spun out
its research into Polymer Vision, and a number of other high tech companies
All the companies working on organic semiconductors have identified flexible
active matrix display backplanes as a potential big market for their
technology, and both Plastic Logic and Polymer Vision now have prototype
production lines and together with partner companies will be in commercial
production of flexible displays by 2007.
The main companies involved have demonstrate flexible active-matrix monochrome
electrophoretic displays based on solution-processed organic transistors
on 25- m-thick flexible plastic substrates. The displays can be bent
to a radius of 1 cm without significant loss in performance.
The great advantage of organic semiconductors is that compared to conventional
silicon semiconductors they are relatively easy to make, and therefore
relatively cheap to manufacture. This is because the circuits can be
fabricated using well understood and conventional printing technologies
at room temperature and using cheap lightweight plastic substrates, as
opposed to the high temperature furnaces, vacuum chambers, and expensive
silicon wafers used in silicon semiconductor manufacture.
One way to further reduce costs is to integrate (part of) the display
drive circuitry, such as row shift registers, directly on the display
substrate. Using the same process flow Philips have developed row shift
registers. With 1,888 transistors, these are the largest organic integrated
circuits reported to date. More importantly, the operating frequency
of 5 kHz is sufficiently high to allow integration with the display operating
at video speed. This work therefore represents a major step towards 'system-on-plastic'.
Although silicon chips will initially be used to drive organic electronic
backplanes, in the longer term we will see integrated row and column
drivers, shift registers and decoders constructed using organic electronics.
For most display applications, organic electronics can be efficiently
used as drivers. At a clock rate of one kilohertz, it is possible to
change all of the information in a 1,000-by-1,000 element display in
one second. With clever electronics this can be reduced to a fraction
of a second. (note this is much quicker than loading a Web page using
a conventional computer)
Although organic semiconductors, at this moment, have a low component
density, and are very slow. However, neither of these limitations poses
any problem in building active matrix backplanes for flexible displays.
It is an application tailor made for the technology.
Flexible silicon backplane
Of all the backplane technologies the best understood, and capable
of offering the most advanced features, is silicon. Silicon thin film
on a glass substrate has been used for TFT active matrix backplanes in
LCD screens for many years. They can be manufactured with high reliability
using processes that are well known and generically similar to those
used in IC manufacture.
Glass substrate silicon backplanes are used in most of the first generation
e-paper displays now currently in use. They are used in most signage
products and will work with most frontplane technologies. A typical example
is the Sony LIBRIe e-book reader with its 800x600 monochrome E-Ink frontplane
The problem with silicon thin film on a glass substrate as a display
backplane is that it is rigid, heavy, and being made of glass very fragile.
This largely precludes this type of backplane being used in e-paper displays
since they do not live up to many of the points in our definition.
However, recent research has shown that it is possible to put silicon
thin-film circuits, including display backplanes, onto a lightweight,
flexible plastic substrate. The resulting backplanes certainly live up
to the definition of e-paper, and although commercially they are in the
more distant future the technology needs to be considered within the
scope of this report.
Electrochromic materials are materials that change colour upon application
of an electrical potential. Electrochromic systems have been used successfully
in the past in mirrors and windows for anti-glare and anti-reflective
applications. However, other potential applications have yet to reach
the market, largely due to the limitations of existing electrochromic
To date, most commercial electrochromic technologies have used solution
based systems, which rely on organic electrochromic species dissolved
in the electrolyte compartment of an electrochemical cell which has at
least one transparent electrode.
The most commonly used electrochromophores, salts of 4,4'-bipyridines,
also called viologens, are synthetically tunable which allows for different
colours, and have intrinsically high extinction coefficients, yielding
excellent colouration intensities. The switching speed depends on the
diffusion of these and other redox active species in the electrolyte
to the electrodes and is typically in the order of seconds.
Because the redox active species are dissolved in an electrolyte these
mobile molecules will diffuse to both electrodes once an appropriate
electrical potential has been applied to the circuit. Once the potential
has been removed, the charged species mix, transfer their charges, and
the colour dissipates from the system. Therefore there is no open circuit
memory in these devices and power must be applied continuously to maintain
The term OLED stands for Organic Light Emitting Diode and as this implies
the technology is based upon the use of special organic compounds, light
emitting polymers, that emit light when electricity is passed through
them. The technology was developed back in the 1980s at the Cavendish
laboratory at the University of Cambridge.
The technology of Light Emitting Polymers or LEPs allows the construction
of full colour displays that are much cheaper to make and run than CRTs
because the active material is plastic. Like the CRT, LEP is an emissive
technology, meaning that it emits light as a function of its electrical
operation. An LEP display consists solely of the polymer material manufactured
on a substrate of glass or plastic and does not require additional elements
such as the backlights, filters and polarizers typical of LCDs.
LEP is a platform technology that will scale from tiny devices literally
millimeters in dimension to large high-definition devices that could
be up to a couple of metres.
The only real drawback to OLED technology for use in portable battery
powered devices is that because it is an emissive technology it uses
a lot more power than most reflective technology displays, neither does
it have the power saving feature of bistability shown by many other technologies
covered in this report.
It is also limited in its appeal to devices designed for use in dim ambient
light; since it is almost impossible to see what is displayed on an OLED
display when viewed in bright sunlight.
Whilst in theory OLED displays can be constructed on thin flexible plastic
substrates, all of today's commercial OLED displays use rigid glass substrates.
This is partly because it has been easier to construct TFT active matrix
backplanes on glass substrates using standard LCD manufacturing equipment,
but it is also because of the fact that the polymers used in OLEDS are
quickly destroyed by exposure to air and moisture.
This means that means that they need to be very well encapsulated to
protect them, and to date a reliable encapsulation technique for flexible
substrate OLED displays has not been developed to a commercial stage.
However, research into ways of solving this problem is ongoing and researchers
have expressed the belief that it will probably be solved within the
next year. In the meantime the only reliable way of encapsulating OLED
displays is with a rigid substrate like glass.
The phenomenon of electrowetting offers the possibility of creating
flexible displays with a far faster refresh rate than is possible with
electrophoretic displays, making such displays suitable for displaying
video content. The colour displays will also be much brighter than conventional
LCD displays since they will not rely upon filters.
This technology could be regarded as very similar in principle to electrophoretic
display technology, but without several of its drawbacks. It is still
in the development stage, but should be in commercial production by 2010.
Each pixel of the display consists of a micro cell that contains a drop
of coloured, oily ink suspended in an aqueous medium. At the bottom of
the cell is a reflective white background that has been coated first
with a transparent material that conducts electricity - permitting electrical
control of the pixel colour - and then with a transparent film of a water-repellent
Normally the ink droplet will spread across the entire pixel thus obliterating
the white background. However, if a voltage is applied, the oil droplet
retracts, much like a bead of water in a Teflon pan, thereby exposing
the white area below.
If the microcell is small enough, these white and ink covered regions
are not individually visible. Instead, the effect is that the pixel acquires
just an average brightness level, so that when the droplet is fully spread,
the pixel looks dark, and when it retracts, the pixel looks much lighter.
A very significant feature of this technology is the fact that the larger
the applied voltage, the more the ink droplet retracts. This means that
the ink capable of a continuous grey scale, not just of the bichromal
contrast found in most electrophoretic technologies. This feature will
remove much of the
"jaggies" or roughness due to digitisation, and will produce
images that look very smooth.
The key to the system's success is its switching voltage. It is low enough
that controlling the electronic ink requires only a small power source.
Switching between dark and bright states takes only about ten milliseconds
- fast enough to produce sharp video images.
In principle full-colour images might be produced with this technology
by using four sub-pixels inked with the standard yellow-cyan-magenta-black
quadruplet system. However, this technology is not bistable, requiring
an applied voltage to maintain the image.
Until quite recently many in the technology industry were uncertain
whether organic electronic backplane technology would become a commercial
reality in the near future. At least one company has taken a bet that
it would not, and developed an ingenious micro-electromechanical alternative.
Whilst it looks as if the bet did not pay off, the technology may nevertheless
prove to be quite successful in some applications, in particular large
A touchscreen is technology that accepts direct onscreen input. This
can be done by either an external light pen or an internal device, a
touch overlay and controller, that relays the X,Y coordinates of the
fingertip or pen to the computer. For bendable e-paper displays the most
likely form of touchscreen input is the resistive touchscreen.
Resistive touchscreens are a well developed technology and are widely
used on CRT and LCD screens where they form a transparent overlay that
is attached to the surface of the display and to the control electronics.
This technology should work on all semi-rigid/flexible e-paper displays,
and since the transparent touch film transmits 85% of the light it should
not significantly degrade display quality and readability.
Resistive touchscreen displays rely on a touch overlay, which is composed
of a thin flexible top layer and a more rigid bottom layer separated
by insulating dots. The inside surface of each of the two layers is coated
with a transparent conductive coating of Indium Tin Oxide that creates
a resistance gradient across each layer when voltage is applied.
Pressing the flexible top sheet with a fingertip or a pen creates an
electrical contact between the resistive layers, producing a switch closing
in the circuit. The control electronics alternate voltage between the
layers and pass the resulting X and Y touch coordinates to the touchscreen
controller. The touchscreen controller data is then passed on to the
computer operating system for processing.
Resistive touchscreen technology possesses many advantages over other
alternative touchscreen technologies (acoustic wave, capacitive, Near
Field Imaging, infrared). It is highly durable, and less susceptible
to contaminants that easily infect acoustic wave touchscreens. In addition,
resistive touchscreens are less sensitive to the effects of severe scratches
that would incapacitate capacitive touchscreens, they are also more cost-effective
than Near Field Imaging touchscreens.
Although resistive touchscreen technology exists in 4-wire, 5-wire, or
8-wire forms, 8-wire resistive technology is the preferred form because
of its benefits over its counterparts. Whereas 8-wire touchscreens are
available in all sizes, 4-wire resistive technology is restricted to
small flatpanels (<10.4"). In contrast to 5-wire resistive touchscreens,
8-wire touchscreens do not experience spacer dots and Newton rings. Additionally,
8-wire resistive touchscreens are not susceptible to problems caused
by high-level short-term variances and axis linearity and drift.
At the moment Philips Polymer Vision has said that it is actively developing
a touch screen version of the flexible organic electronic/e-ink display.
No details have been given by the company about input resolution, but
it was implied that both virtual keyboard and handwritten input would